Description
An optical atomic clock based on a single aluminum ion is holding the current world record for accuracy with a fractional frequency uncertainty below $10^{-18}$ [1]. This outstanding precision allows for novel applications like relativistic geodesy [2,3] on the cm level and helps to tighten the bounds for physics beyond the standard model [4]. But single ion clocks are impeded by their low signal-to-noise ratio and require therefore long averaging times. These can be significantly reduced by probing at the coherence time limit of the atom, given by the excited state lifetime. However, such second-long probe times can only be achieved with lasers offering correspondingly long coherence time.
Here we present the current status of PTB’s laboratory aluminum ion clock and the experimental implementation of a multi-ion Ca$^+$ reference based on dynamical decoupling to extend the clock laser coherence time in the future.
We present the estimated error budget of the $^{27}$Al$^+$ clock based on calibration measurements using a $^{40}$Ca$^+$ ion as a sensor. This includes characterization of black body radiation, residual kinetic energy from excess micromotion as well as second order and ac Zeeman shifts caused by dc magnetic fields and magnetic fields from the trap drive, respectively. Recent upgrades of the vacuum system improved the estimated accuracy to $1.1 \times 10^{-18}$.
Improving the signal-to-noise ratio of the clock interrogation requires increasing the number of stored ions or longer interrogation times. We present a continuous dynamic decoupling scheme [5] to suppress inhomogeneous broadening by quadrupole and tensor ac Stark shifts in multi-ion $^{40}$Ca$^+$ crystals. Simultaneous suppression of first order Zeeman shifts allows interrogation of the crystal close to the natural lifetime limit. A clock laser pre-stabilized to a multi-ion $^{40}$Ca$^+$ reference will extend the laser coherence time and enable seconds long interrogation of $^{27}$Al$^+$ in the future, thus forming a compound clock.
[1] S. M. Brewer, J.-S. Chen, A. M. Hankin, E. R. Clements, C. W. Chou, D. J. Wineland, D. B. Hume, and D. R. Leibrandt, $^{27}$Al$^+$ Quantum-Logic Clock with a Systematic Uncertainty below $10^{-18}$, Phys. Rev. Lett. 123, 033201 (2019).
[2] W. F. McGrew et al., Atomic Clock Performance Enabling Geodesy below the Centimetre Level, Nature 564, 87 (2018).
[3] T. E. Mehlstäubler, G. Grosche, C. Lisdat, P. O. Schmidt, and H. Denker, Atomic Clocks for Geodesy, Rep. Prog. Phys. 81, 064401 (2018)
[4] M. S. Safronova, D. Budker, D. DeMille, D. F. J. Kimball, A. Derevianko, and C. W. Clark, Search for New Physics with Atoms and Molecules, Rev. Mod. Phys. 90, 025008 (2018).
[5] N. Aharon, N. Spethmann, I. D. Leroux, P. O. Schmidt, and A. Retzker, Robust Optical Clock Transitions in Trapped Ions Using Dynamical Decoupling, New J. Phys. 21, 083040 (2019).
| Presenter name | Lennart Pelzer |
|---|---|
| How will you attend ICAP-27? | I am planning on in-person attendance |
